A method that has been proposed in recent years for measuring the concentration of a specific gas in a gas is laser adsorption spectroscopy, which uses absorption of wavelength-variable laser light (see for example Patent Literature 1). With this method, a sample cell where the gas to be measured is introduced is irradiated with laser light with a predetermined wavelength and the laser light that is transmitted is analyzed to determine the concentration of a specific gas in the gas based on the amount of absorption. Because, with this device, the light reception unit serving as a sensor does not contact the gas to be measured, the device offers a number of advantages including extremely short response time and the ability to perform measurements without disrupting the sample field.
Among infrared absorption spectroscopy that uses laser light such as the afore-described, spectroscopy that uses harmonic detection such as second harmonics is known as a detection method with a particularly high sensitivity (see for example Non-Patent Literature 1). The theory behind the detection method described in Non-Patent Literature 1 is briefly described next using, as an example, the detection of minute concentration of water vapor in nitrogen gas.
If the sample gas is at atmospheric pressure, the shape of the absorption property is represented by a Lorentz profile, and the relationship between water vapor concentration and the detected intensity of the received laser light is represented by equation (1) below.
Equation 1
                              log          ⁡                      (                                                            I                  0                                ⁡                                  (                  v                  )                                                            I                ⁡                                  (                  v                  )                                                      )                          =                  c          ×          L          ×          S          ×                                    γ              L                                      π              (                                                                    [                                          v                      -                                              v                        0                                                              )                                    2                                +                                  γ                  L                  2                                            ]                                                          (        1        )            Here, I0(ν) represents the intensity of incident light at frequency ν, and I(ν) represents the intensity of transmitted light at frequency ν. c represents the volume concentration of water molecules, L the length of the optical path passing through the gas to be measured, and S the predetermined linear strength of absorption property, and γL the half-width of the absorption property, which is determined by the type of sample gas, temperature and pressure. ν0 represents the center frequency for the frequency modulation.
Equation (2) below represents the absorption intensity I(ν0) at the center frequency.
Equation (2)
                              log          ⁡                      (                                                            I                  0                                ⁡                                  (                  v                  )                                                            I                ⁡                                  (                  v                  )                                                      )                          =                  c          ×          L          ×          S          ×                      1                          πγ              L                                                          (        2        )            
Infrared absorption by water molecules in very low total pressure regions (high vacuum regions where the total pressure of the gas to be measured is less than 1 Torr) results in the width of the absorption property to be narrower than the width of the aforesaid Lorentz profile by a factor of several fold to several dozen fold. The width of the absorption property in said total pressure region is primarily determined by the Doppler effect. The shape of the absorption property is represented by a Gaussian line shape, and the relationship between the detected intensity of the received laser light and water vapor concentration is represented by equation (3) below.
Equation 3
                              log          ⁡                      (                                                            I                  0                                ⁡                                  (                  v                  )                                                            I                ⁡                                  (                  v                  )                                                      )                          =                  c          ×          L          ×          S          ×                      1                                          γ                ED                            ⁢                              π                                              ×                      1                                          exp                ⁡                                  (                                                            v                      -                                              v                        0                                                                                    γ                      ED                                                        )                                            2                                                          (        3        )            
In equation (3), γED is referred to as the Doppler width and depends on the center frequency of the absorption frequency, molecular weight and temperature. Here, the absorption intensity I(ν0) at center frequency ν0 is represented by equation (4) below.
Equation 4
                              log          ⁡                      (                                                            I                  0                                ⁡                                  (                  v                  )                                                            I                ⁡                                  (                  v                  )                                                      )                          =                  c          ×          L          ×          S          ×                      1                                          γ                ED                            ⁢                              π                                                                        (        4        )            
Under conditions of a high vacuum and room temperature of approximately 25° C., with an absorption spectrum in a region of relatively strong absorption that allows the use of an ordinary near-infrared semiconductor laser, γED is approximately equal to 0.01 cm−1. With water molecules that are present in air or nitrogen matrix at 1 atmospheric pressure, the general value of γ is 0.1 cm−1.
Performing harmonic detection requires modulation of the frequency of light that is irradiated onto the gas to be measured. Letting “a” represent the modulation amplitude of the sine wave signal for frequency modulation and w represent frequency, the frequency of light at time t is defined by equation (5) below.
Equation 5ν mod(t)=ν+a·cos ωt  (5)
With second harmonic detection, signal components that correspond to twice the frequency or 2ω are extracted. The second harmonic detection signal at center frequency ν0 for water molecules that are present in air or nitrogen at 1 atmospheric pressure is defined by equation (6) below.
Equation 6
                                          signal            ⁡                          (                              v                0                            )                                            I            0                          =                  c          ×          L          ×          S          ×                      2            π                    ×                                    ∫              0              π                        ⁢                                                            cos                  ⁡                                      (                                          2                      ⁢                      θ                                        )                                                                                                              (                                                                        a                          ⁢                                                                                                          ⁢                          cos                          ⁢                                                                                                          ⁢                          θ                                                γ                                            )                                        2                                    +                  1                                            ⁢                                                          ⁢                              ⅆ                θ                                                                        (        6        )            
Similarly, the second harmonic detection signal at center frequency ν0 for water molecules in a vacuum atmosphere is defined by equation (7) below.
Equation 7
                                          signal            ⁡                          (                              v                0                            )                                            I            0                          =                  c          ×          L          ×          S          ×                      2                                          γ                ED                            ⁢                              π                                              ×                                    ∫              0              π                        ⁢                                                            cos                  ⁡                                      (                                          2                      ⁢                      θ                                        )                                                                                        exp                    ⁡                                          (                                                                        a                          ⁢                                                                                                          ⁢                          cos                          ⁢                                                                                                          ⁢                          θ                                                                          γ                          ED                                                                    )                                                        2                                            ⁢                                                          ⁢                              ⅆ                θ                                                                        (        7        )            These equations are proposed in Non-Patent Literature 2, which also proves that signal (ν0) with the highest sensitivity is obtained when the modulation amplitude a is selected so that a/γ (or a/γED)=2.2 in equations (6) and (7).
The afore-described harmonic synchronous detection method has the advantage of high sensitivity but also has the problem of a narrow dynamic range of sensitivity. To explain, if the concentration of the gas to be measured is low, an accurate detection result can be obtained, but if the concentration of the gas to be measured becomes high, signal intensity becomes saturated, and accurate results cannot be obtained. For this reason, if the concentration of a specific gas in a gas to be measured has to be continuously measured and if the variation in concentration of the specific gas is large, there is a risk that the measurement range will be exceeded.